Chromatic test target



Feb. 13, 1940. R, BElTEL, JR 2,190,008

CHROMATIC TEST TARGET Filed March 29, 1937 ]I T i 9 INVENTOR P055127 J 551751. J?

1T Patented Feb. 13, 1940 T r "rear .c *r egg This invention relates to tests employed in connectionwith the examination of a patients eyes todetermine the correct prescription for ophthalmic lenses required for the patients tions the yellow portion of the spectrum i s sharply gfocusedin the plane of theretina, while, the green portion of the spectrum is sharply focused in a plane inirontofthe retina and the to correct vision. The present invention relates more particularly to a test knownas the bi chrome test. k I l l I An object of the present iinvention is to render the testing of the patients eyeslmore effective W and more accurate.

In the accompanying drawing which illus .trates my inventiony '1" j,

Fig. I illustrates in perspective "atestin Which a display comprising indicia is exhibited to the patient; and

Fig.II is a diagrammatic view illustrating the reaction of the .eye to different colors.

The chromatic test as at present'used aiTords sufficient examination to permit the examiner to approximately determine the correction required for one or both eyes of the patient. By means 01 test lenses, the vision, of the patientfisap proximately corrected in accordance with. this red is ,sharplyfocusedin a plane in the rearjof. the retiriajthe focal planes of the red and green being substantially equidistant from the retinal plane. i 1 Kg In II itlwill be noted that the light rays 2 coming from the green test character 3 wil1 be focused at l in frontof the plane ofthe retinal screen 5 and that theflight rays ii coming from the red testcharacter 7 willbe focused atf in the rearfof the plane of the retinal 'screenl;

The test lens it is illustrated by the dash lines. 13

To check the accuracy of "the prescriptive lenses hefore the eyes a test chart it having .red and green test characters l and 3 respectively thereonis exposed tothe vision of the patient i (see Fig.1). The patient is asked to state which. of the two "test characters appears sharper. Should he report thatthe red test character is the sharper, the examiner immediately knows examination and test characters are exhibited thatlthe eyes areslightlyover corrected. While :3 to the patient to determine the correctness of on the other hand, if he states that the green .25

compensation brought about by the test lenses. is the sharper the examiner immediately knows In giving the chromatic testiaccordingto present that the patient is slightly under corrected.

practice. the characters, which thepatient .views- The chromatic orfbichrome test can be used,

are blackand part of the backgrodridieired and therefore, as a final check in determiningfac t the other part is green.- 1 curately. the prescription [for corrective 'ophthal- This is based upon the characteristic oi the mic lenses. v j ,I

human eye known as chromatc aberrationQ The. characters can be exhibited in any.suit- The chromatic test, otherwise known as the bi able manner. For instance, 1113.945! usea chart chrome test, takes advantage of the difierent:

l2 Whose characters appear on a neutral grey iocal'planes of the different colors which result background. Thus the green characters ,3 are from the chromatic aberration. The test de-' printed or in other manner 'formedon the neutral pends, for accurate results, upon determination grey background on f one side ofjthe chart and by the patient of the relative sharpness of. the red characters 7 ar'eIformed on 'the same backhlack characters upon the red and green background on the other side of the chart. lristead grounds. I have discovered that. thenature of of such a chart .I mayemploy screen and by 40 the background is a factor in the excellence of means of a projector, throw the desired characthe comparison. ter's together with a neutral grey background In accordance with my invention I improve upon the screen If the screen itself constitutes the accuracy of such testing of the eyes by exa neutral grey background, the colored charachibiting to the patient colored indicia on a su ters may be thrown'on the screen Which'thus 5 stantially uniform background. If. the test is provides a uniform background for these. charthe above mentioned chromatic or bichrome test,

acters. V I employ red and green characters onsuch a Instead'of an achromatic background, I may background. According to one embodiment of display the red and green characters on a monomy invention, I employ red and green characters chromatic yellow background; preferably a on an achromatic background, such a neutral monochromatic yellow. approximating the maxi grey background. According to another emmum of the visibility functionof the eye with I bodiment of my invention, Iemployred and respectto wave length. This maximum isabout green characters on'a yellow background. at the sodium line in the spectrum; and corre' 7 As illustrated in Fig. II, under normal ccndisponds' to the ,wave length of light used to compute indices of refraction of lens glass and for making other optical calculations, and is there-- fore an advantageous wave length to use in causing the eye to focus.

To further increase the accuracy of the test the shades or tones of the red and green test characters and background may be so controlled and illuminated that the two characters andbackgrounds will be of substantially equal intensities so that there will be greater ease in comparison on the part of the patient.

From the above it, therefore, will be seen that the procedure is substantially as follows: The patient is first tested in the usual manner by displaying suitable test charts within his field of I vision and by placing corrective test lenses before the field of vision ofthe patient.

his eyes which are varied in power untilthe patient reports clear vision-of the test characters.

,When the best correction has been found in this 1 manner, a chart or other arrangement for displaying colored test characters is placed within 7 One eye is occluded andthe patient is then asked which of the two colors, namely, the red or the green the colored test characters has-a uniform. background and that this uniformity equates the backgrounds for the red and for the green test characters, a more accurate test of this nature It will be apparent from the above discussion of my invention that I have changed the stimulation of the eye in such test by making the characters of a color rather than making them black, i. e. absence of color. In the chromatic or 'bichrome test as at present'practiced, the eyeattempts to 1 make a sharp focus of the boundaries of the characters which are black. If the red or green background is in focus, the contrast of black to red or green along the edgeof the character is sharp, and a clear impression of the character is made on the eye.

In accordance with my invention, the eye focuses to a large extent on the red or green characters themselves to the exclusion of the background. Thisis what the test aims to accomplish, since it is the difierence in the focal planes of the different colors which are relied upon to give an accurate test. In accordance with my invention, the background for the red character is the same as the background for the green char- 'acter; hence, the uniform background balances in the cases of the red and of the green letters,

may be obtained than has been usual in the past.

and if this background gives some slight stimulation to the retina along borders of the red character or indicia, the same stimulation is given to the retina along the edges of the green character or indicia. As nearly as possible, therefore, I limit the test to the difference between stimulation of the eye by red characters and stimulation of the eye by green characters in the chromatic or bichrome test.

It will be understood that while I have illustrated and described a specific manner of practicing my invention, the principles may be otherwise embodied'and practiced within the scope of the following claims.

I claim:

1. In a device of the character described for testing the visual acuity of an eye, a test chart having abackground of a color which is of uniform intensity and wavelength frequency composition throughout its area and which is essentially of the color having the wavelength frequency of that portion of the visible spectrum which approximates the maximum of the visibility function of a normal eye, and stimulus objects of diiferent color wavelength frequency compositions which are difierent from that of the background by substantially equal amounts, said stimulus objects being of distinguishable color sensationand of such frequencies that one is less and the other greater than the frequency of the background, said background and test characters being of such a nature as to cooperate with each other to afford equal stimulation of the retina of the eye under test by each of said test characters so as to take advantage of the chromatic aberration of the eye as a means of determining the clarity and definition of said test characters relative to each other.

2. In a device of the character described for testing the visual acuity of an eye, a test chart having a yellow background which is of uniform intensity and wave length frequency composition throughout its area to form a background'having the colonfrequency of that portion of the visible spectrum which approximates the maximum of the visibility function of a normal eye,

and stimulus objects of diiferent color wavelength frequency compositions, one of which is red and the other green normally different from the yellow by similar amounts whereby, in a normal eye, the yellow will focus in the plane of the retina of the eye and the red and green objects will be off focus similar amounts, one in front of the retina of the eye and the other in the rear of the retina of the eye with the intensity of said objects providing equal stimulation of the retina of the eye under test so as to take advantage of the chromatic aberration of the eye as a means of determining the clarity and definition of said test characters relative to each other.

ROBERT J. BE-ITEL, JR. 

